The present invention relates to a car stereo provided with a detachable grille, and more particularly to a system for preventing the erroneous operation of a display when detaching the grille.
Increase in the use of sophisticated car stereo in recent years has caused an increase in the amount of theft of these stereos. In order to prevent the theft of the stereo, there has been many proposals. In accordance with one of the proposals, a detachable grille having a plurality of push buttons for control of the stereo is attached to the body of the stereo.
FIG. 1 shows the above described prior art. The car stereo comprises a car stereo body 10 and a detachable grille 20 attached to the body 10. The body 10 has recessed panel 11 mounted at the front thereof. The panel 11 has a cassette opening 14. A detector switch 15 is provided on an upper left hand side of the grille 20 to mechanically detect the attachment of the grille 20 on the body 10. A connector 12 which is connected to a connector 21 (FIG. 2) provided at the back of the grille 20 when the grille is attached, are formed on the panel 11. The grille 20 has a cassette opening 24 which coincides with the cassette opening 14 of the body 10 and various operation keys 22. A display 23 is disposed at a central portion of the grille 20 so as to indicate information such as current time and the received frequency of the stereo.
Referring to FIGS. 2 the connector 12 of the body 10 has a 5 V supply terminal 12a, data input/output terminal 12b, 8 V supply terminal 12c, and ground terminal 12d. The terminals 12a and 12c are connected to a 5 V supply circuit 17 and an 8 V supply circuit 18, respectively, through a protection circuit 16. The data input/output terminal 12b is connected to a microcomputer 19. The microcomputer 19 is connected to the detector switch 15 so as to operate the protection circuit 16 in accordance with the operating state of the switch 15. More particularly, when the grille 20 is detached, the switch 15 is opened, so that the protection circuit 16 stops the supply of voltage from the 5 V supply circuit 17 and 8 V supply circuit 18 to the terminals 12a and 12c. Thus, the car stereo circuitry provided in the body 10 is protected from damage which is caused by either the short-circuiting of the exposed terminals 12a to 12 d or by static applied to the terminals.
The connector 21 of the grille 20 has terminals which correspond to the terminals 12a to 12d of the connector 12. Namely, there is provided a 5 V input terminal 21a, data input/output terminal 21b, 8 V input terminal 21c, and ground terminal 21d. The terminal 21a is connected to a microcomputer 25 so as to supply a 5 V voltage thereto. The microcomputer 25 receives data from the microcomputer 19 through the data input/output terminals 12b and 21b dependent on the operation of the operation keys 22 on the grille 20, such as the time and the frequency of the radio. The microcomputer 25 applies a control signal to a display driver 26 to indicate the received data on the display 23. The 8 V input terminal 21c is connected to a lamp 27 for lighting the display 23.
When the grille 20 is attached to the body 10, the terminals 21a to 21d of the connector 21 are connected to the respective terminals 12a to 12d of the connector 12. At the same time, the detector switch 15 is closed, so that the microcomputer 19 operates to release the operation of the protection circuit 16. Accordingly, the 5 V supply circuit 17 and the 8 V supply circuit 18 are connected to the microcomputer 25 and the lamp 27, respectively, thereby operating them. The microcomputer 25 is applied with various data from the microcomputer 19 through the terminals 12b and 21b, so that, the microcomputer 25 operates to drive the display 23, thereby indicating information on the display 23.
On the contrary, when the grille 20 is detached, the detector switch 15 is opened. As a result, the microcomputer 19 renders the protection circuit 16 operative, so that the supply of the voltage to the terminals 12a and 12c is stopped. The microcomputer 19 also stops feeding the data to the microcomputer 25. Thus the display 23 of the grille 20 does not show information.
In such a device, it may occur that the contacts of the terminals 21a to 21d with the terminals 12a to 12d chatter when the grille is detached. If the detector switch 15 is opened before the contact chattering, or the power input terminal 21a is removed from the terminal 12a before the chattering of the contacts between terminals 21b and 12b, there is no trouble, because of the reset of the microcomputer 19 or 25 due to the stop of the power supply. However, if the contact of the terminal 21b with the terminal 12b chatters under the connection between the terminals 12a and 21a, random signals are supplied to the microcomputer 25, which may cause an erroneous display or flickering of the display 23.